Conventionally, an electric power steering apparatus detects the steering torque applied to a steering wheel by a driver, and computes a target steering assist torque corresponding to the detected steering torque, and controls the supply of electricity to a motor so as to obtain the computed target steering assist torque. In general, the electric power steering apparatus calculates a target current which is proportional to the target steering assist torque, detects the current actually flowing through the motor by use of a current sensor, and applies to the motor a voltage corresponding to the difference between the target current and the current actually flowing through the motor. That is, the electric power steering apparatus drives and controls the motor while performing feedback control.
In the case where the current sensor has suffered a failure, feedback control cannot be performed. Patent Document 1 proposed the electric power steering apparatus which assists steering even in such a case where the current sensor has suffered a failure. The electric power steering apparatus proposed in Patent Document 1 changes its control mode from feedback control to open loop control when a current sensor failure is detected. That is, the electric power steering apparatus controls switching devices of a motor driver circuit at a duty ratio set such that a voltage whose magnitude is proportional to the steering torque is applied to the motor.